The Big Squeeze

“I want a bunch of horsepower, I want it now, and I don’t want to spend very much to get it.” If that’s your definition of wretched excess, then look no further than nitrous oxide. Street guys call it squeeze, fast gas, spray, or “NOS” (“naus” if you endured the movie, The Fast and the Furious). But no matter what you call it, this is supercharging in a bottle—the quick way to go fast. As with any go-fast performance component, there’s plenty of tech info that’s accompanied with its fair share of misinformation. This story will be CHP’s version of the old Jack Webb TV show, Dragnet—we’re going to deal with “just the facts, ma’am.”

Chemistry 101

Let’s start at the molecular level. Nitrous oxide consists of two nitrogen atoms combined with one oxygen atom (N2O). Nitrous oxide is an oxidizer, but it will not burn if subjected to a flame. However, it does an outstanding job of contributing to the combustion process and making horsepower. Early experimenters quickly learned that when compressed in a tank above 760 pounds per square inch (psi), the gaseous nitrous turns into a liquid. When this liquid N2O is released, the lower pressure transforms the liquid nitrous back into a gas but reduces its temperature at the same time, dropping the nitrous to -127 degrees F.

This is the first advantage of nitrous oxide. Before the oxygen can contribute to combusting fuel in the cylinder, just releasing the gas into the intake manifold radically reduces the inlet air temperature as much as 65 degrees, which can be worth about six percent in additional power. At this point, in the intake manifold, nitrous is still a combined molecule. Once it finds its way into the combustion chamber and is subjected to a temperature of more than 572 degrees F, the chemical bond that holds the oxygen and nitrogen atoms together disassociates, and that single oxygen element is free to combust more fuel.

The Ratio Game

This brings up an extremely important point. Engines burn fuel, which in our case is gasoline. Engines make power by ingesting air and then mixing the appropriate amount of fuel with the air. For normally aspirated engines, the ideal power ratio is 12.5-13:1 air/fuel ratio. A richer mixture (such as 11.5:1) with excess fuel does not increase power. In fact, it reduces power. A rich mixture makes less power because most of the free oxygen atoms have combusted the fuel with a slight amount of fuel and air remaining since combustion is never 100 percent efficient. The point here is that power is determined by the amount of air you can shove into the cylinder.

Injecting nitrous into the engine adds a greater percent of oxygen because nitrous is 36 percent oxygen by weight. The air we breathe is 78 percent nitrogen, 21 percent oxygen and another 1 percent other gases. Adding this greater oxygen content requires additional fuel to maintain that safe 12.5:1 air/fuel ratio for best power. In fact, all nitrous systems are intentionally jetted far richer to 11:1 or sometimes more. A rich air/fuel ratio also reduces the possibility of a too-lean cylinder that could cause a melted piston.

The advantage of nitrous is that the nitrogen acts as a combustion buffer, slowing the resultant pressure rise that occurs when the oxygen-enhanced air/fuel charge is ignited. This is why N2O is a far better choice than injecting pure oxygen. With pure oxygen, the resulting combustion process would be so violent that it would immediately destroy the engine. Even at the same power level, nitrous creates a much quicker cylinder-pressure rise than either a normally aspirated or supercharged engine. This is the big reason why nitrous engines demand less ignition timing. Optimal cylinder pressure should occur with the piston at roughly 10 to 15 degrees after top dead center (ATDC) to take maximum advantage of the leverage offered by the crankshaft as cylinder pressure pushes down on the piston. This is why over-advanced ignition timing, even in a normally aspirated engine, makes less power because the ignition begins the combustion process so early that the engine is working against cylinder pressure as the piston approaches TDC.

We mentioned that additional fuel must be added whenever nitrous oxide is introduced into the engine. If additional fuel is not included, the combustion temperature rises radically and can actually achieve a temperature hot enough to melt a forged aluminum piston. This is the beginning of what is commonly called “catastrophic engine failure.” The obvious lesson is to avoid lean mixtures when running nitrous. Trash heaps are full of burned pistons sacrificed on the alter of nitrous education.

Basics

There are very specific ways to introduce nitrous into an engine. For the same reason you don’t dump fuel from a ½-inch hose into an engine, you also must be careful how nitrous is introduced into the engine. Because nitrous bottles usually operate between 900 and 1,000 psi, an electric solenoid is employed as a valve to connect nitrous pressure to the engine. On the fuel side, the solenoid does not have to be as large or as electrically powerful since it operates against a much lower (5 to 9 psi) line pressure in carbureted applications.

So what we end up with are three separate circuits for a typical nitrous system. These are the nitrous side, the fuel side, and the electrical circuit. Most current nitrous systems employ jets to control the amount of nitrous and fuel. The fuel delivery side operates much like the nitrous side, also using a solenoid to introduce fuel to be mixed with the nitrous. The most overlooked circuit is the electrical side of the solenoids. If the nitrous solenoid fails to open, nothing much is hurt except that the engine runs very rich. However, if the electrical system fails to open the fuel solenoid when the nitrous is triggered, the engine will run dead lean in a micro-second, which can cause serious damage. This is why paying close attention to the electrical circuit is so important.

For first-time nitrous users, the plate system is the easiest and best way to go. The least expensive systems use a dedicated restriction to determine the amount of nitrous and fuel. For slightly more money, you can purchase adjustable systems that provide a range of horsepower increases, generally from 125-175 hp. These systems come with nitrous and fuel jets that are carefully matched to prevent a lean condition. It’s best to always use these factory-established jetting combinations, especially when you’re first getting started.

With time, you may want to increase the horsepower of the system. This may eventually lead you to multiple stage systems where the power can be introduced in layers. For example, a two-stage system could start with a 150 hp shot to move the car off the starting line. Then, using a timer, the second stage can be introduced about 2 seconds into the run. This may add another 100 hp to produce a total of 250 hp. The advantage of staging the nitrous is that hitting the rear tires with 250 hp worth of nitrous on the starting line would only create instant tire smoke. Staging the nitrous makes it easier to maintain traction and improve acceleration.

Systems Approach

There is much more to nitrous than just a bottle, a button, two solenoids, and a plate. The two other systems critical to safe, continuous nitrous operation are a high-output ignition system and a high-volume fuel delivery system. Of the two, the fuel delivery system is absolutely critical. All entry-level nitrous systems tap into the stock fuel delivery system to feed the gasoline solenoid. This places an additional load on the fuel system. The best way to judge if the existing fuel pump can handle the load is to perform a dynamic test before using the system for the first time.

This test requires an accurate fuel pressure gauge mounted on the outside of the cockpit where it can be viewed under acceleration. Install the entire nitrous system but leave the nitrous bottle closed. Make an acceleration run with the nitrous system on and watch the fuel pressure gauge. Because no nitrous is flowing, the engine will run rich, but this places the appropriate load on the fuel system. As long as the system maintains a minimum of 4.5-5.0 psi of fuel pressure, the system will work just fine.

If the fuel pressure drops below 4.5 psi during the run, this indicates that a larger-capacity pump is necessary to ensure adequate fuel delivery. This is a crucial step and not one that should be overlooked. This test involves a little more work but will prevent engine damage from insufficient fuel pressure. It’s actually a good idea to mount a permanent fuel pressure gauge so that you can always monitor fuel pressure.

Greater cylinder pressure is the source of more horsepower. What many enthusiasts overlook is that this additional pressure also places a greater demand on the ignition system. As cylinder pressure increases, greater ignition voltage is required to jump the spark-plug gap. Reducing the gap is one way to make it easier for the ignition system to light the nitrous charge. >> A good CD ignition system from ACCEL, Crane, Jacobs, Mallory, or MSD is always a good idea, especially if you add 200 hp or more of the fast gas.

Of course, playing with big loads of nitrous also requires some careful planning. For example, with plate nitrous power levels above 175 hp, Edelbrock recommends using a single-plane intake manifold because dyno-testing has shown that it is difficult to control mixture distribution in a dual-plane intake manifold, which can lead to burnt pistons.

We’ve only really just scratched the surface of the nitrous world in this story. Nitrous is by far the quickest, easiest, and least expensive way to make more horsepower. Sure, you’ve got to refill the bottle, but the total cost is still far less than a blower or turbochargers. As long as you respect the system and don’t do anything dumb, nitrous is fun, fast, and affordable. All you have to do is hit the button and hang on!